Apparatus for measuring body fat

ABSTRACT

A body fat measuring apparatus for measuring body fat in a patient&#39;s body comprises apparatus for simultaneously measuring an impedance between the patient&#39;s feet, and for measuring the patient&#39;s height and the patient&#39;s weight, and a calculator for calculating body fat from the measured impedance value, height and weight. The calculator estimates body density using a formula having a correction term either for increasing the body density upon increase of the impedance compared to the original impedance or a correction term for decreasing the body density upon increase of the weight with respect to the original weight.

This is a Continuation-in-Part application of Ser. No. 07/948,606, filedon Sep. 23, 1992, now abandoned.

BACKGROUND TO THE INVENTION

1. Field of Invention

This invention relates to a method and apparatus for measuring body fatfor measuring the amount of fatty tissue in vivo.

2. Description of the Prior Art

It has been customary to determine whether or not a human being is fatfrom the relationship between body weight and height.

However, it is not always unhealthy that weight is large in comparisonwith height, and it is important to measure fatty tissue in vivo. It ispractical to measure body fat in vivo by measuring body weight.Recently, various body fat measuring instruments have been sold in themarket.

An underwater body weighing method is known for estimating body fat bycalculating body density from the measured weight value under water as amethod for accurately measuring body fat in vivo.

This underwater body weighing method needs a large facility, andnecessitates skillful measuring techniques and an effort by the personbeing measured considering the large influence of the amount of residualair in the lungs to be measured.

In order to eliminate the disadvantages of the above-describedunderwater body weighing method, there has been proposed a method forestimating body density by measuring body impedance between extremitiesof the body and estimating the body density from the measured impedancevalue, height and body weight.

The ratio of body water in fat-free tissue of body composition tissue isconstant and the specific resistance of the fat-free tissue is constant.The estimation formula: body density=1.1554-0.0841*weight*impedance/(height)² of Segal et al., and body formula:density=1.1303--0.726*weight*impedance/(height)² of Naka et al., byobtaining the inference formula as represented by bodydensity=A-k*weight*impedance/(height)² and obtaining correlation betweenthe constant A and the value obtained actually by an underwater bodyweighing method for determining the constants A and k, are disclosed.

However, it is known that the impedance value varies due to variousfactors such as variation in body water ratio in fat-free tissue due tovariation in body weight, amount of blood plasma due to variation inmotion or attitude and movement of interstitial liquid. In order toaccurately measure the impedance value, it is necessary to considerinfluences of ingestion of food and water, motion, attitude andvariation in body weight.

Thus, the impedance value of a body to be measured vary according to thedate, time and differences of body condition before measurement. Thesevarious are too large to use the above-described inference formulaaccurately and amount of body fat calculated by the formula is likely tobe inaccurate.

Therefore, in order to measure accurately, it is necessary to maintainthe body state of the patient to be measured constant by selecting a dayfor taking the measurements when ingestion of food and water, andactivity, are limited and variation in body weight is small, and tomeasure the impedance value after the patient has been lying on a bedfor a predetermined period of time. Thus, the measuring conditions areknown.

On the other hand, there has also been proposed a method for calculatingbody fat from numeric values with respect to a body such as an impedancebetween extremities of the body, height, weight and gender by measuringthe impedance using a 4-terminal Kelvin bridge living body impedancemeter as a method for accurately and simply measuring the body fat invivo as in U.S. Pat. Nos. 4,008,712 and 4,895,163 (Japanese PatentLaid-open No. Hei 2-60626).

Since the degrees of bending and the angle of the limbs influence theimpedance values using these methods, the measuring conditions when thepatient is lying on a bed are maintained constant to eliminate errors.

Further, if the spacing between a current supply terminal and a voltagedetecting terminal of electrodes for detecting the impedance is notsufficient, the measured values are affected, and hence the spacing ismaintained by mounting the current supply terminals on the backs of thehand and foot and mounting the voltage detecting terminals on thetendons of the wrist and the ankle.

Accordingly, restrictions in the measuring position are significant(needs a bed, etc.), and it is difficult for a person to use theapparatus to take measurements of himself without assistance.

Moreover, in a method for measuring living body impedance, it has alsobeen necessary to measure body fat as described above, using a methodhaving steps of outputting a sine wave of 50 kHz from an oscillator as aconstant-voltage source, converting it to a constant current of 800microamperes by a voltage/current converter, supplying the currentthrough a pair of electrodes mounted on a body, outputting the voltagevalue of a voltage drop from a pair of electrodes mounted inside theabove-described electrodes by a differential amplifier,waveform-shaping, rectifying it, DC-converting it, then, A/D-convertingit as digital data, and applying it to a calculator, determining theliving body impedance Z by Z=V/I from the living body current I and theterminal voltage V, and measuring the voltage V when the current I isconstant, thereby determining the living body impedance Z.

However, the constant-current source is affected and its output isvaried due to the influence of the external environmental temperature,etc., by the degree of variation in the electrode impedance, and it isdifficult to obtain an accurate value of the living body current I as anaccurate constant current, thereby causing error in measuring the livingbody impedance.

On the other hand, the living body impedance to be measured by a bodyfat meter needs to be accurate in a wide range of 0 to about 1 kilo-ohm.It is extremely difficult to obtain an ideal proportional relation inthis range, thereby necessitating a great deal of work for makingcorrections to the measured value.

Accordingly, an object of this invention is to provide a method andapparatus for measuring body fat which has a small restriction inmeasuring conditions and which can be simply measured without influenceby the measuring conditions.

Another object of this invention is to provide a practical body fatmeasuring apparatus which can be readily used by the person beingmeasured without restriction.

Still another object of this invention is to provide a living bodyimpedance measuring method which is particularly adapted for measuringbody fat and can accurately measure the absolute value of the livingbody impedance in a wide measuring range without influence of theenvironment.

SUMMARY OF THE INVENTION

According to an aspect of this invention, there is provided a method formeasuring body fat by the steps of measuring impedance betweenextremities of a body, estimating body density from the measuredimpedance value, measuring the height and the weight, and calculatingthe body fat, comprising the step of estimating a body density by acalculation formula having one of a correction term for increasing thebody density upon increase of the impedance with respect to only theimpedance and a correction term for decreasing the body density uponincrease of the weight with respect to only the weight.

According to another aspect of this invention, there is provided a bodyfat measuring apparatus for measuring body fat in a patient's bodycomprising a measuring station for simultaneously measuring an impedancebetween extremities of the patient's body, a patient's height and apatient's weight, and a calculator for calculating body fat from themeasured impedance value, height and weight, whereby the body density isestimated by a calculation formula having one of a correction term forincreasing the body density upon increase of the impedance with respectto only the impedance and a correction term for decreasing the bodydensity upon increase of the weight with respect to only the weight.

According to still another aspect of this invention, there is provided abody fat measuring apparatus having the steps of measuring an impedancebetween extremities of a body, and calculating body fat from numericvalues of physical conditions such as the measured body impedance value,height, weight, and gender comprising a pair of conductive electrodesfor contacting the patient's toes defined as current supply terminals ata nearest interval of 5 cm or more, and a pair of electrodes forcontacting a patient's heels defined as voltage detecting terminals onan insulating base having protrusions for heel guides in such a mannerthat, when the patient stands thereon, the electrodes are mounted toprotrude from the base symmetrically with respect to the width of thebase spaced apart for the patient to stand in an upright position sothat the patient's feet are not touching and the patient's bodyimpedance can be measured in the standing position.

According to still another aspect of this invention, a method formeasuring impedance of a person is provided which comprises the steps ofinserting a plurality of reference resistors in a current path forsupplying a current to a living body, measuring voltage drops of thereference resistors by momentarily switching the reference resistorgroup and the living body to obtain a correlation formula between themeasured value by the reference resistors and the impedance, anddetermining the impedance of the living body by the patient's measuredvalue and the correlation formula.

This invention will now be described in further detail with respect topreferred embodiments as illustrated in the accompanying drawings.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a schematic perspective view of a body fat measuring apparatusshowing an embodiment of the present invention;

FIG. 2 is a plan view showing, in detail, a base of a body weighingapparatus of FIG. 1;

FIG. 3 is a sectional view taken along the line 3--3 of FIG. 2; and

FIG. 4 is a block diagram showing an example of a circuit arrangementfor carrying out a living body impedance measuring method according tothe present invention.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

Referring to FIG. 1, a body fat measuring apparatus is shown as anembodiment of the present invention. This apparatus comprises anelectronic body weighing meter 5 having a base 1 on its upper surface,an electronic height meter 4, and a display unit 6. This electronic bodyweighing meter 5 electronically calculates a patient's body weight whena patient stands on the base 1. The electronic height meter 4 detectsthe height of a patient standing on the base 1 using a detector 4A andelectronically calculates the height value.

As shown in FIGS. 2 and 3, the base 1 is formed of an insulatingmaterial, and a pair of conductive electrodes 3 for a patient's heelsand conductive electrodes 2 for a patient's toes are mountedsymmetrically with respect to the width of the base spaced to allow thepatient to stand upright so that the patient's feet are not touchingwhen the patient stands thereon on both feet. The electrodes 2 and 3 areset to 5 cm or more apart so as to eliminate the influence of apotential distribution. The electrodes 2 are formed in an ellipticalshape so as to correspond to various sizes of a patient's foot. Theelectrodes 2 are used as current supply electrodes, and the electrodes 3are used as voltage detecting terminals.

Heel guides 7 are provided at the rear of the electrodes 3 so that auser can always stand at a predetermined position on the base 1 with hisheels in contact with the electrodes.

An electronic circuit for carrying out a body impedance measuring methodaccording to the present invention is placed in the housing of theelectronic body weighing meter 5, and an example of the electroniccircuit is shown in a block diagram of FIG. 4. In FIG. 4, electrodes Acorrespond to the electrodes 2 for the patient's toes in FIGS. 1 to 3,and electrodes B correspond to the electrodes 3 for the patient's heelsin FIGS. 1 to 3.

The body impedance measuring electronic circuit in FIG. 4 measures apatient's body impedance when a patient stands on the base 1 of theapparatus in FIG. 1 in such a manner that the patient's toes and heelsare contacted correspondingly with the electrodes 2 (A) and theelectrodes 3 (B).

An oscillator 11 generates a sine wave of 50 kHz as a constant-voltagesource, which is converted to a constant current of 800 microamperes bya voltage/current converter 12, which does not affect influence tolinearity of a body impedance. The constant current is supplied from apair of electrodes A through a plurality of reference resistors 12, 14,15 having known resistances to cover an average measuring range (a rangenecessary to measure an impedance between the extremities of a patient'sbody as individual differences: 0 to 1 kilo-ohm).

Values of voltage drops of the reference resistors 13, 14, 15 in agroup, and values of voltage drops of the pair of electrodes B mountedinside a living body from the electrodes A are momentarily switched by aswitch 16 to be controlled by a microprocessor unit (MPU) 21 whichincludes a calculator, and differences therebetween are output by adifferential amplifier 17. The voltage value output by the differentialamplifier 17 is rectified by a rectifier 18, waveform-shaped by a lowpass filter (LPF) 19 to be DC-converted, A/D-converted by ananalog/digital converter (AD) 20, and input as digital data to themicroprocessor unit 21 obtains a correlation formula between themeasured value of the reference resistor and the impedance, anddetermines a living body impedance from the measured value of thepatient's living body and the correlation formula.

An electronic circuit for using in a body fat measuring method accordingto the present invention is also associated in the housing of theelectronic body weighing meter 5. This body fat measuring electroniccircuit calculates a patient's body fat as below from the patient'sweight measured by the electronic body weighing meter 5, the patient'sheight measured by the electronic height meter 4 and the patient's bodyimpedance calculated by the above-described body impedance measuringelectronic circuit.

An inference formula of body density (DB) of Segal et al., using weight(W), height (Ht) and impedance (Z), is represented by:

    DB=A-k*W*Z/Ht.sup.2                                        (1)

where

A is a constant,

k is a proportional coefficient

The value proportional to the weight and the impedance Z and inverselyproportional to the square of the height is subtracted from the constantA.

However, as described above, it is known that the impedance value variesdue to various factors such as variation in body water ratio in fat-freetissue due to variation in weight, blood plasma due to variation inmotion or attitude and movement of interstitial liquid in a shortperiod. The variation in the impedances greatly influences the bodydensity in the formula (1), but the variation factor of the impedancedoes not originally greatly vary the body density.

In order to reduce the influence of the variation in the impedances tothe body density, a correction term for reversely affecting theinfluence of the variation in the impedances of the formula (1) to thebody density, i.e., a correction term for increasing the body density ifthe impedance is increased is provided as a third term, and a correctedreference formula is represented by:

    DB=A-k'*W*Z/Ht.sup.2 +k"*Z                                 (2)

where k' and k" are proportional coefficients.

On the other hand, the above-described assumption is based on thepremise that the variation in the weight is small with respect to thevariation in the impedance, and it can be corrected if the variation inweight is responsive to the variation on the measured impedance. It isreplaced by providing the correction term for performing the sameoperation as the influence of the variation in the weight in the formula(1) to the body density, i.e., the correction term for reducing the bodydensity if the weight is increased as a third term, and a correctionformula is represented by:

    DB=A-k'*W*Z/HT.sup.2 -k"*W                                 (3)

In order to reduce measuring conditions of an impedance between apatient's feet, with the patient in a standing attitude, and toaccurately measure the impedance, tests were conducted by a number ofsubjects under various conditions to obtain correlation between theabove-described assumption and an underwater body weighing method, andmost desirable coefficients in the formulae (2) and (3) have beendecided. The coefficients A, k' k" may be determined experimentallyand/or statistically so that the equations provide measures of bodydensity which most closely approximate those measured by means of anunderwater body weighing method.

The following formula could be obtained as a most suitable inferenceformula for estimating a patient's body density by using the impedancevalue between both of the patient's feet in a standing position based onthe formula (2):

    Body density=1.1144-0.0976*W*Z/Ht.sup.2 +0.000084*Z

where

W: body weight (kg)

Z: impedance (ohm)

Ht: height (cm)

On the other hand, the following formula could be obtained as a mostsuitable inference formula for estimating a patient's body density byusing the impedance value between both of the patient's feet in astanding position based on the formula (3):

    Body density=1.1442-0.0626*W*Z/Ht.sup.2 -0.00044*W

where

W: body weight (kg)

Z: impedance (ohm)

Ht: height (cm)

Body fat can be obtained by the following formula:

    body fat (%)=(4.57/body density-4.142)*100

The correlation to the underwater body weighing method of the result forcalculating the body fat by the above-described inference formula is ofthe same degree as that for fixing the measuring conditions without acorrection term, the variation due to the difference of the measuringconditions is reduced to 1/2 of that without the correction term,thereby obtaining a satisfactory result.

According to the present invention, body fat can be concurrentlymeasured in a standing position with extremely small error in themeasurements by correcting the inaccuracy of the measured values due tothe variation in the impedance value according to the measuringconditions.

Further, the body fat measuring apparatus according to the presentinvention can readily be used by a user to measure his body fat byhimself merely by standing upright at a designated position, and he canrepeatedly measure it under the same conditions and can also take themeasurements in the standing position. Therefore, restrictions in themounting position of the measuring instrument are remarkably reduced.

We claim:
 1. A body fat measuring apparatus for measuring body fat in apatient's body comprises a measuring station for measuring impedancebetween the patient's feet and an operating circuit coupled to saidmeasuring station, said measuring station comprising a base on which apatient can stand, a pair of patient heel electrodes and a pair ofpatient toe electrodes mounted on said base, constant current supplyingmeans connected between the patient toe electrodes for supplying aconstant current between the patient toe electrodes and voltagemeasuring means connected between the patient heel electrodes formeasuring a voltage between the patient heel electrodes, the patientheel electrodes and the patient toe electrodes being positioned on thebase so that bottoms of the toe and the heel of one foot of the patientand bottoms of the toe and the heel of the other foot of the patientcontact each of the electrodes mounted on said base, respectively, whenthe patient stands on the base, said operating circuit comprising meansfor calculating the impedance (Z) of the patient from the value of theconstant current flowing between the patient toe electrodes and thevalue of the voltage between the patient heel electrodes, means forcalculating the body density (DB) of the patient from the calculatedimpedance (Z) of the patient, a patient's height (Ht) and a patient'sweight (W) according to the following equation:

    DB=A-k'*W*Z/Ht.sup.2 +k"*Z

where A is a constant, k' is a proportional coefficient, and k" is aproportional coefficient,and means for calculating the body fat of thepatient from the calculated body density.
 2. A body fat measuringapparatus as defined in claim 1 wherein the measuring station furthercomprises means for measuring the patient's height and means formeasuring the patient's weight.
 3. A body fat measuring apparatus formeasuring body fat in a patient's body comprises a measuring station formeasuring impedance between the patient's feet and an operating circuitcoupled to said measuring station, said measuring station comprising abase on which a patient can stand, a pair of patient heel electrodes anda pair of patient toe electrodes mounted on said base, constant currentsupplying means connected between the patient toe electrodes forsupplying a constant current between the patient toe electrodes andvoltage measuring means connected between the patient heel electrodesfor measuring a voltage between the patient heel electrodes, the patientheel electrodes and the patient toe electrodes being positioned on thebase so that bottoms of the toe and the heel of one foot of the patientand bottoms of the toe and the heel of the other foot of the patientcontact each of the electrodes mounted on said base, respectively, whenthe patient stands on the base, said operating circuit comprising meansfor calculating the impedance (Z) of the patient from the value of theconstant current flowing between the patient toe electrodes and thevalue of the voltage between the patient heel electrodes, means forcalculating the body density (DB) of the patient from the calculatedimpedance (Z) of the patient, a patient's height (Ht) and a patient'sweight (W) according to the following equation:

    DB=A-k'*W*Z/Ht.sup.2 +k"*W

where A is a constant, k' is a proportional coefficient, and k" is aproportional coefficient,and means for calculating the body fat of thepatient from the calculated body density.
 4. A body fat measuringapparatus as defined in claim 3 wherein the measuring station furthercomprises means for measuring the patient's height and means formeasuring the patient's weight.